Aqueous-based polyurethanes historically have not approached the performance of solvent-based polyurethanes. In attempts to overcome this deficiency, the chemical industry has formulated two-component dispersions intended to capture through the properties of the individual polymers more of the properties of the solvent-based dispersions.
One formulating technique for making two component dispersions consists in blending a dispersion of a polyurethane with a dispersion of a second polymer. If the polyurethane is hydrophobic it will be difficult to disperse; therefore, it is first dissolved in a water-miscible solvent, which acts as a dispersing aid. The polyurethane in solvent is then dispersed with water through a phase inversion utilizing high shear forces and surfactants. The problem with this method is that the solvent eventually must be distilled off, raising costs and environmental considerations. Moreover, when this polyurethane dispersion is blended with another polymeric dispersion, the blending itself is subject to chemical, thermodynamic and kinetic limitations. When more than two different polymeric species are formulated, the blending process becomes more difficult and complex, and can result in phase separation, large particle size, the use of high levels of dispersing aids, high viscosity and non-homogeneous dispersions or gelation/coagulation.
Another technique, which avoids the use of organic solvents and blending processes, consists of making a dispersion of an interpenetrated polymer of a polyurethane and a polymer formed by vinyl addition polymerization. A polyurethane prepolymer is dissolved in the monomers for the vinyl addition, and this mix is then dispersed with water. The resulting aqueous dispersion is subjected to vinyl-addition polymerization conditions to polymerize the monomer and form the interpenetrated polymer. This technique, however, relies on the use of a hydrophilic polyurethane. Polyurethanes can be made hydrophilic and self-dispersing by the incorporation into the polymer of ionic or amphoteric moieties, or of nonionic moieties derived from polyethylene glycol. The use of anionic or cationic moieties on the polyurethane causes pH instability. For anionic polyurethanes, the pH should be above 6.5, and for cationic polyurethanes, the pH should be below 7.5. Moreover, hydrophilic polyurethanes have reduced water and solvent resistance and are undesirable for some applications.
Thus, each of these methods has limitations. If the polyurethane is hydrophobic, accomplishing the dispersion requires the use of a water miscible organic solvent and surfactants to accomplish the dispersion. This, consequently, means the subsequent removal of the organic solvent. In the case where monomer is used as a reactive diluent and later polymerized to form the interpenetrated polymer, the art has taught that the polyurethane needs to be hydrophilic. When hydrophilic moieties are incorporated into the polymer, both solvent resistance and water resistance in the final product are inferior to those properties in solvent-borne polyurethanes.
These problems create a need for dispersions of hydrophobic polyurethanes, and particularly for interpenetrated polymers prepared from hydrophobic polyurethanes and polymers prepared from ethylenically unsaturated monomers. There also exists a need to provide dispersions having a broader range of performance properties than can be achieved by dispersions made from current processes, particularly through dispersions that can incorporate polymeric performance enhancers. These needs are met by the instant invention.